Previous studies have shown that it can help to keep fat off, but doesn't help to lose it, though one study showed an increase of 0.8 lb per month. However, recent studies have shown potentially serious side effects. Every one I know that was taking it has stopped.
According to an article in the NY Times
Researchers have found that it can increase blood levels of C-reactive protein, lipoprotein and leptin — all of which can heighten the risk of heart disease. There is also some evidence that taking C.L.A. daily increases insulin resistance, a sign of impending Type 2 diabetes.